1. Field of the Invention
The present invention relates to a technique for improving signal-to-noise (S/N) performance of an imaging device in a shooting under low-intensity light conditions. More particularly, it relates to a compact imaging device preferably usable in combination with a portable terminal such as an information device or a portable phone.
2. Description of the Background Art
FIG. 14 illustrates the structure of an inline transfer CCD generally used as a solid state imaging element (prior art). In FIG. 14, reference numeral 100 denotes a photosensitive section for performing photoelectric conversion of brightness of a subject image (incident light), numeral 101 denotes a vertical transfer section for reading charges generated and accumulated at the photosensitive section 100, and transferring the same in a vertical direction, and numeral 102 is a horizontal transfer section for transferring the charges of a picked-up image signal vertically transferred from the vertical transfer section 101 in a horizontal direction, and taking out the same as a picked-up image signal. Further, reference numeral 103 collectively denotes vertical transfer clocks φV1, φV2, φV3 and φV4 for driving the vertical transfer section 101, and numeral 104 collectively denotes horizontal transfer clocks φH1 and φH2 for driving the horizontal transfer section 102.
FIGS. 15A through 15F illustrate an exemplary timing chart of the transfer clocks in the inline transfer CCD given in FIG. 14 (prior art). The accumulated charges read out into the vertical transfer section 101 from the photosensitive section 100 are transferred in a vertical direction (in a direction of a downward-pointing arrow in FIG. 14) to be timed to the vertical transfer clocks φV1, φV2, φV3 and φV4 at a starting point of a signal transmission period 6. As a result, the accumulated charges at a bottom end of the vertical transfer section 101 are vertically transferred to the horizontal transfer section 102.
To be timed to the horizontal transfer clocks φH1 and φH2, the accumulated charges in the horizontal transfer section 102 are transferred in a horizontal direction (in a direction of a leftward-pointing arrow in FIG. 14), and then outputted as a picked-up image signal to an external CDS circuit (not shown). This CDS circuit thereafter performs sampling on the signal of the received accumulated charges.
FIG. 16 is a block diagram illustrating the configuration of an imaging device introduced in Japanese Patent Application Laid-Open No. 2000-224473. In this device, incident light passing through a lens 105 is converted into a video signal by a CCD 106. The video signal is then sampled by a CDS circuit 107 and inputted to a low-intensity light detecting circuit 115. When a subject is in low-intensity light conditions, the low-intensity light detecting circuit 115 outputs a low-intensity light signal. An ALC circuit 114 controls an aperture of the lens 105. After passing through an AGC circuit 108, the video signal is converted into a digital video signal by an A/D converting circuit 109. A signal combining circuit 110 combines three or more digital video signals corresponding to adjacent pixels, and thereafter, performs division in such a manner that a resultant gain is more than 1. When the subject is in low-intensity light conditions, the signal combining circuit 110 outputs the result therein in response to the control by a gain-up control circuit 116. A signal processing circuit 111 performs predetermined signal processing on the video signal. After passing through a D/A converting circuit 112, the video signal is then outputted from an encoding circuit 113.
As described, the conventional imaging device has a signal processing circuit using a signal combining circuit for combining adjacent pixels and a divider circuit. As a result, the conventional imaging device has a problem involving reduction in S/N ratio on a displayed image obtained by a shooting under low-intensity light conditions.
Further, the conventional imaging device performs gain-up control of a digital signal obtained after A/D conversion, thus resulting in a problem of loss of bit information.